Field of the Invention
The present invention relates to an optical wavelength demultiplexer, and more particularly to an optical wavelength demultiplexer for polarization-independently performing wavelength selection suitable for use in a wavelength multiplexing communication system and the like.
Description of the Background Art
Recently, there has been put into practical use a telecommunications network system having the configuration of an access optical communication called Fiber-To-The-Home (FTTH), in which subscriber lines are directly led into users' private premises in the so-called last one mile from a central-office facility. One of the advantages of employing the FTTH system is that it can provide communication services and video distribution services with a single optical fiber line.
In communication services, the passive optical network (PON) system, inter alia, the GE-PON (Gigabit Ethernet-PON) communication system is becoming a mainstream, which communicates over an access section at an ultra-high speed of 1 Gbit/s according to the PON scheme, “Ethernet” being a trademark. The PON communication system is of a form of communication system which connects, by optical fiber lines and an optical star coupler, an optical line terminal (OLT) disposed in a central office and optical network units (ONUs) disposed in respective subscriber premises and in which the OLT is shared by the plural ONUs. In the GE-PON communication system, Internet connection services and IP (Internet Protocol) telephone services are provided by establishing IP data communications in which a wavelength of 1310 nm is assigned to uplink data signals and a wavelength of 1490 nm is assigned to downlink data signals.
For video distribution services, for example, many of CATV (Community Antenna Television) service providers are also employing the FTTH system. In video distribution services through CATV and the like, video signals are distributed on a wavelength band of 1550 nm.
In the FTTH system, a signal light beam of the wavelength band of 1550 nm assigned to a video distribution service and a signal light beam of wavelengths of 1310 nm and 1490 nm assigned to a communication service are multiplexed with each other at an OLT to be distributed to ONUs. Therefore, the ONUs installed in subscriber premises utilizing the FTTH system require optical wavelength demultiplexers to separate the signal light of the wavelength band of 1550 nm and the signal light of the wavelengths of 1310 nm and 1490 nm from each other. Through the optical wavelength demultiplexer, the signal light assigned to the video distribution service and the signal light assigned to the communication service are split from each other.
In the ONUs or the like of FTTH systems, optical wavelength demultiplexers are constituted by wavelength demultiplexing devices, for example, WDM (Wavelength Division Multiplexing) filters. In manufacturing such demultiplexers, a technology using a silicon-based material as a waveguide material has attracted attention since it is excellent in mass productivity and miniaturization. This technology implements a silicon thin line waveguide as a wavelength demultiplexing device.
The silicon thin line waveguide is structured such that a silicon core is enclosed with a clad made of a material with its refractive index lower than the refractive index of the silicon core. The silicon thin line waveguide can strongly confine a light beam to be guided into the core since the refractive index difference between the silicon core and the clad can be set to be extremely large. Accordingly, it is possible to implement fine waveguides whose core dimension is in the order of submicron and hence to miniaturize wavelength demultiplexing devices such as a WDM filter formed therefrom.
Various optical wavelength demultiplexers have been practiced using silicon thin line waveguides, and have the basic structure thereof including a plurality of interferometer type wavelength filters utilizing the interference phenomenon of light. As an interferometer type wavelength filter, known is, for example, a wavelength demultiplexing device utilizing light interference, such as a Mach-Zehnder interferometer (MZI) type or a multimode interference (MMI) type device. In particular, wavelength demultiplexing devices of MZI type are commercialized as devices for demultiplexing light signals conveying information of two to four channels separable with respect to the wavelength. Refer to, for example, Wim Bogaerts, et. al., “Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 16, No. 1, (2010), Folkert Horst, “Silicon Integrated Waveguide Devices for Filtering and Wavelength Demultiplexing”, OFC/NFOEC 2010, paper OWJ3, March 2010, and Japanese Patent Laid-Open Publication Nos. 2009-198914 and 2003-149472.
An MZI type wavelength demultiplexing device is typically configured to connect MZI devices in multi-stage through directional couplers to obtain a wavelength separation characteristic intended. However, directional couplers have the coupling coefficient thereof lower for light having its wavelength shorter, and require high dimensional accuracy in order to ensure a level of wavelength selectivity required for recent FTTH systems. Therefore, directional couplers require a very advanced technology for manufacturing.
In the FTTH system, as described above, a wavelength band of 1550 nm is assigned to a video signal of video distribution services and a wavelength of 1310 nm and a wavelength of 1490 nm are assigned respectively to an uplink data signal and a downlink data signal in communication services. However, since interval between those wavelengths is not uniform unlike the WDM wavelength grid, it is difficult for a device for separating wavelengths in an equal interval, such as an AWG (Arrayed Waveguide Grating), to separate those wavelengths from each other. Furthermore, ONUs primarily intended for use in GE-PON communication systems communicating at an ultra-high speed may receive downlink data signals, transmitted from an OLT, including the wavelength component of a 1550 nm band, which is the wavelength band for video signals as far as they are connected to an FTTH system. In order to make an ultra-high speed communication service, such as a GE-PON communication system, available in good condition, it is necessary to sufficiently remove the wavelength component of the 1550 nm band from downlink data signals transmitted from an OLT.
Thus, in FTTH access optical communication networks, a wavelength demultiplexing device to be installed in an ONU is required which can sufficiently remove the wavelength component of the 1550 nm band to the extent that ultra-high speed communication services on the GE-PON system and the like are reliably processed and can separate and extract downlink data signals of the 1490 nm wavelength.
Furthermore, since the polarization of a signal light propagating on an optical fiber line is indefinite, optical wavelength demultiplexers usable in the FTTH system are also required to be operable polarization-independently.